Strain Channel Engineering for increasing channel carrier mobility plays an important role in CMOS integrated circuits with a feature size smaller or equal to 90 nm. Uniaxial technologies for creating stress are integrated into the device technology. The NMOS and PMOS in a CMOS are covered by stress thin films, which are different from each other in property to increase the carrier mobility in the channel. As shown in FIG. 1, an NMOS 2 and a PMOS 3 are formed on a semiconductor substrate 1, and are insulated from each other through an STI. The thin film 5, which covers NMOS 2, has tensile stress, while the thin film 6, which covers PMOS 3, has compressive stress. Usually, the stress thin films include silicon nitride.
The intrinsic stress in a silicon nitride thin film is mainly caused by the intrinsic property of the nitrogen-centered network structure units in a triangle plane, which tends to form a silicon-centered tetrahedral network structure having a low energy valence bond. The different chemical valences between said two kinds of atoms cause strain. SiNxHy with tensile stress is formed by a PECVD process using ammonia-silane as reacting mixture, mainly including the formation of disilane and amino silane group of gas phase, surface reaction of the plasma products, and the subsequent releasing of superfluous hydrogen on the secondary surface through removing reaction of hydrogen and ammonia. The stretched Si—N bond formed in said densifying process will be restricted by the surrounding net texture, and thus will be effectively frozen into a tensile stress state.
The temperature of the substrate in PECVD is lower than that in LPCVD, and hence there is less removing reaction. As a result, the formed thin film contains more Hydrogen composition, the flexibility of the net texture is enhanced and thin film stress is reduced. Therefore, a high temperature surface anneal cure process is required to dehydrogenize and densify the thin film so as to increase its stress. The high temperature surface anneal cure discharges more content of hydrogen element, resulting in higher thin film stress. However, if the temperature is too high, the low temperature advantage and feature of PECVD will be lost, and meanwhile the formed MOSFET silicide and source-drain doping will be affected adversely.
Hence, Ultraviolet Thermal Processing (UVTP) is used to treat the PECVD silicon nitride to increase thin film stress. The energy of the ultraviolet photon helps to break the Si—H bond and N—H bond in the thin film. The hydrogen atoms in adjacent broken bonds is combined to form hydrogen in the form of molecules, which diffuses from the thin film, so that dangling bonds and micropores are formed in the thin film. The dangling bonds cross-link together, so that said micropores contract to minimize surface energy.
The conventional silicon nitride thin film has a small absorption coefficient in a UVTP system, and the substrate needs to be heated to improve the effect of dehydrogenization of the thin film. Therefore, there is a need for a new semiconductor device and manufacturing method, which can obtain a better dehydrogenization effect in the UVTP system without heating the substrate.